In the glass manufacturing process glass sheets (known as float glass) can be combined with different kind of elements, such as coated or tempered layers to create glass panes for different purposes having specific properties. For example, insulating glass units, IGUs, are constructed typically with a configuration having two or more glass sheets with a closed space in between the sheets, where the closed space is filled with gas with low thermal conductivity, such as Argon, Xenon, Krypton Nitrogen or mixture of those. There is need in the industry to check the quality of the glass unit and ensure that there is no leakage, so that the filling gas has not leaked away.
Different kinds of solutions are known from the prior art for determining the quality and possible leakages of a gas mixture contained in the spacing. For example WO 2012/156589 relates to a non-invasive method for determining a concentration of a gas component in a gas mixture contained in a spacing of a glass unit having at least two glass sheets spaced apart from each other and forming said spacing. In addition SMITH C J ET AL: “Real-time calibration of laser absorption spectrometer using spectral correlation performed with an in-line gas cell”, OPTICS EXPRESS, voi. 21, no. 19 (2013 Sep. 17), XP055353294, 001: 10.1364/0E.21.022488 discloses a real-time drift correction and calibration method using spectral correlation based on a revolving in-line gas cell for laser-based spectroscopic trace-gas measurements, and is focused for measuring and especially ensuring accuracy of the concentration measurement over long time.
EP0417884 discloses calibrating a non-dispersive infrared gas analyzer especially adapted for measuring the concentrations of HC, CO and CO2 in a vehicle exhaust is described.
EP2372344 discloses a method for analysing a gaseous component present in a hermetically sealed container, the latter is placed in a measuring station between a laser beam emitter and a receiver. The laser beam is emitted towards the receiver and through the portion of the container, where the gaseous component is located, and the concentration of the gaseous component is inferred by analysing the spectrum of the laser beam absorbed by the gaseous component.
WO2011/007047 discloses a solution where one or more light beams from a light source with defined polarization are directed at a suitable angle to the material surfaces, such as glass panes conveyed on a production line. Reflected beams from the material interfaces are directed through a linear or circular polarizer with defined polarization properties and their positions and intensities are measured while the measurement location is altered. Of the related parameters, the thicknesses of constituent materials are calculated from the reflection positions, interface type from the average reflection intensity and the possible tempering from the intensity fluctuations of the reflections. Also concentration of at least one gas component in a gas mixture contained in a closed spacing between the two layers of said transparent object (for example an insulated glass unit comprising a gas mixture in a closed spacing between the panes in order to minimize heat conduction through the glass unit) can be detected.
U.S. Pat. No. 6,639,678 discloses a system for non-destructive monitoring of gases in sealed containers. The system includes a tunable diode laser (TDLAS) source that provides a uncollimated laser beam for absorption in a substance to be measured. TDLAS determines the concentration of a gas by measuring the amount of light absorbed at a particular wavelength. The intensity of light absorbed is directly related to gas concentration through Beer's law.
US2007/131882 discloses a method of detecting a target gas in a monitored space comprising applying an electrical control current to a laser diode so as to generate optical radiation of a wavelength defined by the control current, transmitting the optical radiation across the monitored space and determining the optical absorption thereof, wherein the control current defines two mean wavelengths ∧1, and ∧2 for the optical radiation and includes electrical modulation at two frequencies f and f′ respectively, wherein ∧1 and ∧2 are respectively close to two separate optical absorption lines of the target gas, and f and f′ are not harmonically related.
Coeola L: “Tunable diode laser absorption spectroscopy for oxygen detection”, PH.D. THESIS IN SCIENZE TECNOLOGIE E MISURE SPAZIALI (2012 Jan. 27), XP055198219, UNIVERSITY OF PADOVA discloses how the traditionallimits of Tunable Diode Laser Absorption Spectroscopy are addressed with digital signal processing techniques and careful optical design towards the realization of gas sensing instruments with the stability, robustness and reliability that are required in an industrial environment.
Typically these solutions are based on measuring concentration of the gas components in the gas mixture contained in the spacing, such as measuring absorption peak of the filling gases and thereby the concentration of the filling gas. However, the concentration measurements of the filling gas has some drawbacks, namely for different types of filling gas a different laser source must be used, which is clearly expensive and time consuming way to measure. Another drawback is that the gas volume inside the typical glass unit is very small, whereupon the amount of the gas to be measured is small and thereby also the absorption peak (amplitude of the peak) caused by said gas component to the measuring beam is very weak. In addition, and therefore, the location of the absorption peak of the gas component to be measured might be very hard to find from the measured signal due to environmental noise, which easily covers the absorption peak to be determined and thus makes the analysis very cumbersome and labour.